Polymer resins are molded into a variety of useful products. One such polymer resin is polyethylene terephthalate (PET) resin. It is well known that aromatic polyester resins, particularly PET, copolymers of terephthalic acid with lower proportions of isophthalic acid and polybutylene terephthalate are used in the production of beverage containers, films, fibers, packages and tire cord. U.S. Pat. No. 4,064,112 discloses a solid-state polycondensation or polymerization (SSP) process for the production of PET resins.
While for fibers and films the intrinsic viscosity (IV) of the resin must generally be between 0.6 to 0.75 dl/g, higher values are necessary for molding materials such as containers and tire cord. Higher intrinsic viscosity such as greater than 0.75 dl/g can be obtained directly through polycondensation of molten PET, commonly called the melt phase process but only with great difficulty and degradation of the product due to the high shear required to move the increasingly viscous melt. The SSP process is a practical and widely practiced technique to drive polymerization to a higher degree and increase the molecular weight of the polymer by the heating and removal of reaction products while in the form of granules or chips. The polymer with a higher molecular weight has greater mechanical strength and other properties useful for production of containers, fibers and films, for example.
An SSP process starts with polymer chips that are in an amorphous state. U.S. Pat. No. 4,064,112 teaches crystallizing and heating the chips in a crystallizer vessel under agitation to a density of 1.403 to 1.415 g/cm3 and a temperature ranging between 230° and 245° C. (446° and 473° F.) before entering into the SSP reactor. Otherwise the tacky chips tend to stick together during the polymerization process. Various reactions occur during polycondensation of PET. The main reactions that increase the molecular weight of PET is the elimination of either ethylene glycol or water:    1. PET-COO—CH2—CH2—OH+HO—CH2—CH2—OOC-PET→PET-COO—CH2—CH2—OOC-PET+HO—CH2—CH2—OH    2. PET-COOH+HO—CH2—CH2—OOC-PET→PET-COO—CH2—CH2—OOC-PET+H2O
An inert gas such as nitrogen is run through the polymerization reactor to strip the developing polymer of impurities. The impurities present in the inert gas stream used in the production of polyethylene terephthalate in an SSP process generally include water and organics such as aldehydes and glycols, typically acetaldehyde, ethylene glycol and glycol oligomers. Also, volatile impurities include low molecular weight PET oligomers, such as the cyclic trimer of PET and other oligomers. Water and ethylene glycol are removed from the inert gaseous stream before it is recycled to the SSP because these materials can reverse the polymerization reaction. All impurities are removed to strengthen the polymer product and to assure that they do not taint the compatibility of the end product with its use. Especially important is the prevention of organic impurities from leaching out of a resin container into the beverage contents. These impurities are stripped from polymer chips and accumulate in the inert gaseous stream. An example of a particularly noxious impurity is acetaldehyde which may be created in trace quantities from the thermal breakdown of end groups of the polymer. Acetaldehyde has a objectionable taste in sensitive commodities such as mineral water and must be removed to typically less than 2 ppm or it will migrate into the food at levels that can be detected by the consumer. The organic impurities are present in the inert gaseous stream leaving the solid stating reaction, in quantities, defined as methane equivalent, of about 2000 to 3000 ppm or more. U.S. Pat. No. 5,708,124 discloses maintaining the ratio of inert gas mass flow rate to PET polymer solids mass flow rate to below 0.6 in an SSP reactor.
It is also well known that polyamide resins, and among them particularly PA-6, PA-6,6, PA-11, PA-12 and their copolymers, find wide application both in the fiber and flexible packaging sectors, and in the manufactured articles produced by blow and extrusion technology. While the resin relative viscosity for fibers is low at about 2.4 to 3.0, higher relative viscosities of 3.2 to 5.0 are needed for articles produced by blow and extrusion technologies. The relative viscosity is increased to above 3.0 by means of an SSP process operating at temperatures of between 140° and 230° C. (284° and 446° F.), depending on the polyamide types used. U.S. Pat. No. 4,460,762 describes an SSP process for a polyamide and different methods to accelerate this reaction.
An SSP process for polyamide resins is also described in the article “Nylon 6 Polymerization in the Solid State,” R. J. Gaymans et al., JOURNAL OF APPLIED POLYMER SCIENCE, Vol. 27, 2513-2526 (1982) which discloses the use of nitrogen as a heating and flushing aid. The reaction is carried out at 145° C. (293° F.).
It is also known that the molecular weight of polycarbonate can be increased through an SSP process. Developing polyamides and polycarbonates also emit organic impurities that must be purged by an inert gas stream that must then be purified.
The SSP process requires that a steady, uninterrupted flow of polymer chips be maintained through the SSP plant. Sticking of polymer chips should be minimized to ensure a smooth flow of chips during the entire SSP process. To this end, the SSP process requires a suitable combination of reactor residence time and temperature of chips to achieve the desired IV while maintaining a desired flow of chips through the plant. Since the reaction rate increases with increasing temperature, and IV increases with increasing residence time, the desired IV can be attained either by a combination of a relatively long residence time with relatively low temperature or the combination of a relatively short residence time with relatively high temperature. However, there are practical limits to the temperature range. Below 190° C., for typical PET copolymers, the reaction rate is quite slow. At a temperature approximately 10° to 30° C. below its ultimate melting temperature, the PET resin begins to greatly soften and must be kept well-agitated or sticking will occur.
It is desirable that the polymer chips obtained in an SSP process have a narrow IV distribution to achieve a narrow molecular weight distribution in the final product. To this end, the flow regime of polymer chips under processing in an SSP plant should be as close as possible to the ideal “plug flow” behavior, in a way that all polymer chips passing through the reactor experience the same process conditions for the same time duration.
The stickiness of the polymer chips is primarily affected by temperature, chip size, reactor height, velocity of flow of chips through the reactor and polymer crystalline morphology. The polymer chips initially moving freely in a moving bed can stick and clot if, for instance, the temperature or bed height is increased or if the rate is decreased. At solid phase polymerization conditions, polyester is only partially crystalline (typically with 25 to 65% crystallinity). As a consequence, such polyester is not a rigid body, but rather, it is leathery and slightly tacky. Since the tackiness of a polymer chip increases with increasing temperature, the sticking tendency of polymer chips also increases with increasing temperature. In a fixed bed of polyester granules held motionless or moving very slowly inside a vertical, cylindrical reactor at polymerization temperatures and under a consolidating pressure due to the weight of the polymer bed, the chips to be polymerized creep into one another at contact points and, in time, the polymer chips will tend to agglomerate and form larger lumps. In severe cases, the lumping and agglomeration may cause bridging of the discharge section of the vertical reactor and interruption of flow. The most effective way to prevent lumping is to constantly renew the inter-chip contact areas so that polymer chips do not have an opportunity to creep into one another. This can be achieved by maintaining a constant flow of polymer chips at a sufficiently high velocity.
Further, since the sticking tendency increases with increasing specific surface area (area per unit mass) or, more precisely, with increasing specific contact area of polymer chips, it also increases with decreasing size of the polymer chips. Reducing chip size tends to accelerate the polymerization process while increasing the tendency for the polymer chips to lump. Thus, while processing small size polymer chips the higher sticking tendency is countered by a reduction in processing temperatures, which, in turn, brings the final values of the process rate back to the typical ones for larger size granules processed at a higher temperature.
The average consolidating force exerted on a particle in a vertical silo is a function of the material properties, wall friction and the vessel diameter. The theory is well-known and approximated by Janssen's formula which is summarized by E. B. Pitman in “Forces on bins: The effect of random friction” in PHYSICAL REVIEW E, Vol. 57, No. 3, March 1998. Applying Janssen's formula moving downward from the top level of the solids in the vertical reactor predicts a rise in pressure to an asymptotic value, usually at a distance about two or three vessel diameters, then a leveling off of pressure. The value of the maximum pressure exerted on the particle increases with increasing diameter. For this reason there are practical limits on the height and diameter dimensions of a solid phase polymerization reactor. At sufficiently high flow velocity, polymer chips change their positions relative to each other (by sliding, for example), and the tendency to form lumps is lessened. Since the rate of changes of contact areas of polymer granules and the reduction in the bulk density of the bed increases with increasing chip flow velocity, polymer sticking tendency within the reactor decreases with increasing chip flow velocity.
Accordingly, in a conventional SSP process, there are two primary methods to incrementally increase the product IV; namely, increasing the reactor temperature or increasing the reactor residence time of polymer chips. The reactor residence time is constrained by bed height and diameter and chip flow velocity. It can be increased by either increasing the bed height or by decreasing the chip velocity. Increasing the reactor diameter allows an increase in the flowrate but not in residence time at constant chip velocity. On the other hand, if the reactor temperature is raised to increase the end product IV, polymer sticking tendency will accordingly increase. One method to prevent polymer sticking would be to decrease the bed diameter and accordingly the force on the particles or to increase the chip velocity. However, if one desires to decreases the bed diameter or increase the velocity, the vessel must be made taller and more expensive to achieve the desired holdup time at a particular reaction rate. If one desires to increase the reaction rate by elevating the temperature, the sticking tendency increases. These constraints limit the ability of conventional plants using vertical single or multiple reactors, to increase intrinsic polymer IV.